Methods and systems for analyzing particles and particularly sediments are well known in the art, as disclosed in U.S. Pat. Nos. 4,338,024 and 4,393,466, which are incorporated herein by reference. Such systems utilize a flow cell though which fluid samples are passed, and a particle analyzer for capturing still frame images of the fluid passing through the flow cell. Thus, the flow cell positions and presents the sample fluid containing particles of interest for analysis. The more accurately that the sample fluid is positioned by flow cell, the better the analysis of the particles therein that can be made.
Typical flow cells cause the sample fluid, and a sheath fluid that buffers the sample fluid, to flow together from a large entry chamber into a small cross sectional examination area or region. The transition from the inlet or entry chambers to the examination region forms a hydrodynamic lens that squeezes both the sample fluid and the sheath fluid proportionally into the smaller space. Where the particles of interest are microscopic particles, the resulting cross-sectional space occupied by the sample fluid must be positioned within the depth of field of the analyzer, such as an optical system or a laser system, to obtain the best analytical information. For the best hydrodynamic focus, a large area of sheath flow must envelop the small area of sample fluid without any swirling or vortices. Thus, uniform flow of sample and sheath fluids through the flow cell is essential for optimal operation of particle analyzers.
Displacement pumps, (e.g. tubing or peristaltic pumps), are well known in the art and have been used to pump fluid samples and sheath fluids through flow cells. Conventional peristaltic pumps include multiple rollers that roll along flexible tubing containing fluid. The rollers push the fluid along the length of the tubing, drawing fluid into an input end of the tubing and forcing fluid out an output end of the tubing. A common configuration includes a rotating hub with rollers on its periphery, and an annularly shaped housing against which the tubing is pressed. With each rotation of the hub, each roller engages with, rolls along the length of, and disengages from, the tubing. At least one of the rollers is in contact with the tubing at all times so that fluid cannot flow backwards through the tubing.
Conventional peristaltic pumps have several drawbacks. For example, multiple rollers engaging with and disengaging from the flexible tube cause pulsations in the fluid flow through the pump, which can be problematic for proper operation of flow cells. Moreover, the amount of fluid delivered by the pump for n degrees of rotation is dependent on the starting angle of the rollers. Most pump designs only retain the tube at its ends, relying on the multiple rollers engaged with tubing to hold it in its circular path along the housing. Thus, the tube can stretch and contract as the rollers move across its length, which again can cause varying flow and uncertainty in the volume moved by rollers. Lastly, when the pump is shut down, rollers are left in contact with the tube, causing compression setting (flat spotting) of the tube, which adversely affects the uniform flow of the fluid after the pump is activated again.
There is a need for a displacement pump that provides uniform fluid flow of known and repeatable quantities, and which does not produce flat spots on the tube during non use.